A process of printing a monochrome image references a one-dimensional LUT (lookup table) or a three-dimensional LUT to convert RGB data (R: red, G: green, B: blue) on a display into CMYKlclm data (C: cyan, M: magenta, Y: yellow, K: black, lc: light cyan, lm: light magenta) and the like used on a printer. The one-dimensional LUT maintains correspondence between gradation value data of the monochrome image as 256 gradations of brightness components and gradation values of CMYKlclm ink. The three-dimensional LUT is similar to an LUT used for the color conversion of ordinary color images. The three-dimensional LUT evenly divides an sRGB to form cubic lattices and maintains correspondence between RGB data and CMYKlclm data at a given lattice point.